The cop was giving a minute-by-minute recap of one of the darkest days in recent U.S. history:

Dec. 14, 2012.

The massacre at Sandy Hook Elementary.

He immediately shoots and kills the principal.

Fullerton PD Sgt. Michael Hines had a rapt audience.

More than two-dozen teachers and staff members at Nicolas Junior High School in Fullerton watched his Power Point demonstration with somber faces as he traced the movements of the killer.

At this point, we’re about three minutes since the first shot was fired. Three minutes is a long time. You know what you can do in three minutes.

Just minutes earlier, classes had been dismissed at Nicolas Junior High.

The killer goes to the bathroom area where 17 kids were and kills every kid in that bathroom except for one.

Hines wasn’t there to scare the teachers and administrators. After all, he said, the odds of being involved in an active-shooter incident are 1 in 250 million.

But it can happen.

And that’s why the Fullerton PD, since March 2013, has been conducting active-shooter training at all of the city’s middle and high schools. And soon, its “Run, Hide, Fight” training will expand to the general public (see information at end of story).

Hines, a 20-year FPD veteran who now runs the agency’s Property Crimes Bureau, is one of several Fullerton cops who’ve been fanning out to schools to present the 45-minute session, which includes a short video produced by Houston First.

The statistics don’t lie:

Between 2007-2013, Hines said, there were an average of 16.4 active-shooter incidents per year —- more than double the average (6.4) of annual incidents between 2000-2006.

“Do you think guns laws are working?” Hines asked. “No.”

One in four active-shooter accidents happen at schools, he said.

Sandy Hook, Hines said, was a “game changer” in training teachers and others how to react to an active-shooting incident (as well as law enforcement).

The old thinking was, much like in the event of an earthquake, get under a desk and hide.

No longer.

Active shooters want to kill as many people in as short amount of time as possible, Hines explained. They are looking for rooms filled with numerous potential victims.

Hines broke down the new strategy of “Run, Hide, Fight.” as outlined in the Houston First video he shared:

RUN

If you can get out, get out — even when others insist on staying. Don’t let the indecision of others slow you down. Remember what’s important — you, not your belongings. Leave them behind. Once you’re out of danger, call 911.

HIDE

If can’t get out if a room or area safely, hide. Act quickly and quietly. Secure a room the best you can. Turn out the lights and silence your cell phones. Try to lock the door, if possible. Do your best to remain quiet and calm. Hide being large objects.

FIGHT

As a last resort, act with aggression. Improvise weapons and commit to taking the shooter down — no matter what. Target vulnerable areas like the eyes, face and groin. “Don’t let up until he’s incapacitated,” Hines said.

Hines noted that 70 percent of mass shootings last less than 5 minutes.That means by the time police and paramedics arrive, most if not all of the damage has been done, he said.

And since it takes about two to five minutes for a wounded person to bleed out and die, school staff members can’t simply call 911 and wait for help, he stressed.

“Who are the true first responders?” Hines asked. “You guys. You have to keep your students alive for three to five minutes until we get here. That’s your job…I’m deputizing all you guys right now. You are my SWAT team.”

Hines offered other pointers to Nicolas staffers:

— Overreact rather than underreact.

— Call 911 from a hardline, when possible.

— Since 85 percent of shooters are students or employees of the school, be aware of warning signs. “The people who are going to go after you likely are the people amongst you,” Hines said. “If you guys see kids acting (suspicious) or think they are dangerous or think they have access to firearms or think they want to go on a killing spree or are writing notes about it, you guys have to take it seriously.”

— Have landlines in classrooms that, with one call, can alert every other classroom in the event of an active shooter.

— Have kits (in addition to standard first aid kits) that include tourniquets, pressure dressings, gloves, wool blankets and other emergency aid materials.

—- If a classroom has a lot of windows, and the killer is coming down the hallway, make sure there is a crowbar or similar item in the classroom to smash the windows and escape. Hines noted this could have saved some victims at Sandy Hook. Instead, the killer was able to slaughter innocents in a confined area.

Nicolas Junior High teachers and staff members were impressed by Hines’ demonstration.

“I really liked it,” said Jeannette Nunez, a special education teacher. “It was really informative, and he made clear what our expectations are.”

Chief Dan Hughes and other members of the Fullerton PD will hold an “Active Intruder” session for the public at 6 p.m. on April 14 at the Fullerton Community Center, 340 W. Commonwealth Ave., Fullerton. For more information, call 714-738-6838.